Copper-tin alloy plating bath

ABSTRACT

An object of this invention is to provide a copper-tin alloy plating bath that allows for film thickening without using cyanide ions, and that can also be applied to barrel plating. This invention relates to a copper-tin alloy plating bath comprising an aqueous solution containing a water-soluble copper compound, a water-soluble divalent tin compound, a sulfur-containing compound represented by formula (1): 
       R—(CH 2 ) l —S—(CH 2 ) m —S—(CH 2 ) n —R  (1),
 
     wherein R is H, OH, or SO 3 Na, and l, m, and n are each independently an integer of 0 to 3,
 
and a hydroxyl group-containing aromatic compound.

TECHNICAL FIELD

The present invention relates to a copper-tin alloy plating bath.

BACKGROUND ART

Nickel plating has been widely used in electroplating. However, there are indications that nickel plating has a nickel allergy problem such that the metallic element (nickel) contained in the plating film causes skin rashes or inflammation. There is thus a need for a technique that replaces nickel plating.

Meanwhile, copper-tin alloys are known to have a white appearance and film properties that are comparable to those of nickel. For this reason, copper-tin alloy plating is drawing attention as an alternative to nickel plating.

Cyanide ion-containing plating baths (cyanide baths) have been used for copper-tin alloy plating, but are problematic in terms of the work environment and wastewater treatment regulations. In recent years, pyrophosphate baths (e.g., Patent Literature 1 to 3), acidic baths (e.g., Patent Literature 4 and 5), and the like have been proposed as cyanide-ion-free (hereinafter may be referred to as “non-cyanide”), copper-tin alloy baths. However, when a pyrophosphate bath is used, compared with the case in which a cyanide bath is used, the formed plating film has high internal stress, and therefore, cracks are generated during plating, thus making it difficult to thicken the plating film. In acidic baths, the deposition potentials of copper and tin are not adjusted; therefore, when an acidic bath is used for barrel plating with a large variation in the current density, copper is preferentially deposited, resulting in a large variation in the alloy composition.

There is thus a demand for a plating bath that enables thickening of a plating film as in the case of a cyanide bath, and that can also be applied to barrel plating.

CITATION LIST Patent Literature

-   PTL 1: JPH10-102278A -   PTL 2: JP2001-295092A -   PTL 3: JP2004-035980A -   PTL 4: JP2009-161804A -   PTL 5: JP2010-189753A

SUMMARY OF INVENTION Technical Problem

The present invention has been accomplished in view of the above-described problems of the conventional techniques. A primary object of the present invention is to provide a copper-tin alloy plating bath that allows for film thickening without using cyanide ions, and that can also be applied to barrel plating.

Solution to Problem

The present inventors conducted extensive research to achieve the above object, and found that a copper-tin alloy plating bath that allows for film thickening without using cyanide ions and that can also be applied to barrel plating can be obtained by using a specific sulfur-containing compound and a specific hydroxyl group-containing aromatic compound. The present invention has been accomplished through further research based on this finding.

More specifically, the present invention provides the following copper-tin alloy plating bath and the like.

Item 1. A copper-tin alloy plating bath comprising an aqueous solution containing a water-soluble copper compound, a water-soluble divalent tin compound, a sulfur-containing compound represented by formula (1):

R—(CH₂)_(l)—S—(CH₂)_(m)—S—(CH₂)_(n)—R  (1),

wherein R is H, OH, or SO₃Na, and l, m, and n are each independently an integer of 0 to 3, and a hydroxyl group-containing aromatic compound. Item 2. The copper-tin alloy plating bath according to Item 1, wherein the water-soluble copper compound is present in an amount such that the amount of copper ions is 1 to 60 g/L, the water-soluble divalent tin compound is present in an amount such that the amount of divalent tin ions is 5 to 40 g/L, the sulfur-containing compound is present in an amount of 5 to 500 g/L, and the hydroxyl group-containing aromatic compound is present in an amount of 1 to 50 g/L. Item 3. The copper-tin alloy plating bath according to Item 1 or 2, wherein the sulfur-containing compound is at least one member selected from the group consisting of methanedithiol, 1,2-ethanedithiol, 1,3-propanedithiol, 3,6-dithia-1,8-octanediol, and bis-(sodium sulfopropyl)-disulfide. Item 4. The copper-tin alloy plating bath according to any one of Items 1 to 3, wherein the hydroxyl group-containing aromatic compound is at least one member selected from the group consisting of phenol, catechol, hydroquinone, resorcinol, pyrogallol, p-cresolsulfonic acid, sodium ascorbate, and sodium erythorbate. Item 5. The copper-tin alloy plating bath according to any one of Items 1 to 4, wherein the aqueous solution further contains a nonionic surfactant, and an aromatic ketone or an aromatic aldehyde. Item 6. A method for copper-tin alloy plating, the method comprising performing electrolysis using an object to be plated as a cathode in the copper-tin alloy plating bath according to any one of Items 1 to 5. Item 7. An article comprising a copper-tin alloy plating film formed by the method according to Item 6.

Advantageous Effects of Invention

Since a specific sulfur-containing compound and a specific hydroxyl group-containing aromatic compound are used in combination in the copper-tin alloy plating bath of the present invention, an alloy film containing copper and tin at any ratio can be obtained. In addition, since a specific sulfur-containing compound is used as a complexing agent in the copper-tin alloy plating bath of the present invention, cracking is less likely to occur compared with the case in which hitherto known pyrophosphate baths are used, and the plating film can be thickened without using a cyanide bath. Further, since, regarding the copper-tin alloy plating bath of the present invention, the current density has a small influence on the alloy ratio compared with the case in which hitherto known acidic baths are used, the copper-tin alloy plating bath of the present invention can also be applied to barrel plating with a large variation in the current density. Moreover, a plating film having an excellent bright appearance can be obtained by further adding a nonionic surfactant, and an aromatic ketone or an aromatic aldehyde to the copper-tin alloy plating bath.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing the relationship between the current density of a copper-tin alloy plating bath and the copper content of a plating film.

DESCRIPTION OF EMBODIMENTS

The copper-tin alloy plating bath of the present invention is described in detail below.

The copper-tin alloy plating bath of the present invention comprises an aqueous solution containing a water-soluble copper compound and a water-soluble divalent tin compound as metal sources, a sulfur-containing compound represented by formula (1):

R—(CH₂)_(l)—S—(CH₂)_(m)—S—(CH₂)_(n)—R  (1),

wherein R is H, OH, or SO₃Na, and 1, m, and n are each independently an integer of 0 to 3, as a complexing agent, and a hydroxyl group-containing aromatic compound.

The water-soluble copper compound, which is a copper ion source, is not particularly limited as long as it is a water-soluble compound containing divalent copper as a copper component. Specific examples of water-soluble copper compounds include copper(II) chloride, copper(II) sulfate, copper(II) nitrate, copper(II) carbonate, copper(II) oxide, copper(II) acetate, copper(II) methanesulfonate, copper(II) sulfamate, copper(II) fluoride, copper(II) 2-hydroxyethanesulfonate, copper(II) 2-hydroxypropanesulfonate, copper(II) pyrophosphate, and the like. Among these copper compounds, copper(II) sulfate is preferable. These water-soluble copper compounds can generally be used singly, or in a combination of two or more. The concentration of the water-soluble copper compound is, for example, such that the copper ion concentration is about 1 to 60 g/L, and preferably about 10 to 40 g/L.

The water-soluble divalent tin compound, which is a tin ion source, is not particularly limited as long as it is a water-soluble compound containing divalent tin as a tin component. Specific examples of water-soluble divalent tin compounds include stannous chloride, stannous sulfate, stannous acetate, stannous pyrophosphate, stannous methanesulfonate, stannous sulfamate, stannous gluconate, stannous tartrate, stannous oxide, stannous fluoroborate, stannous 2-hydroxyethanesulfonate, stannous 2-hydroxypropanesulfonate, and the like. Among these tin compounds, stannous sulfate is preferable. These water-soluble divalent tin compounds can generally be used singly, or in a combination of two or more. The concentration of the water-soluble divalent tin compound is, for example, such that the divalent tin ion concentration is about 5 to 40 g/L, and preferably about 5 to 25 g/L.

The proportions of the water-soluble copper compound and the water-soluble divalent tin compound are preferably such that the copper:tin ratio (metal component molar ratio) is 1:0.1 to 0.6, and more preferably such that the copper:tin ratio (metal component molar ratio) is 1:0.1 to 0.3.

A significant feature of the present invention is that a sulfur-containing compound represented by formula (1):

R—(CH₂)_(l)—S—(CH₂)_(m)—S—(CH₂)_(n)—R  (1),

wherein R is H, OH, or SO₃Na, and l, m, and n are each independently an integer of 0 to 3 is used as a complexing agent. Specific examples of sulfur-containing compounds represented by formula (1) include methanedithiol, 1,2-ethanedithiol, 1,3-propanedithiol, 3,6-dithia-1,8-octanediol, bis-(sodium sulfopropyl)-disulfide, and the like. Among these compounds, for example, 3,6-dithia-1,8-octanediol and bis-(sodium sulfopropyl)-disulfide, both of which have little odor, are preferable from the viewpoint of the work environment, and 3,6-dithia-1,8-octanediol is more preferable. These sulfur-containing compounds can generally be used singly, or in a combination of two or more. The concentration of the complexing agent is, for example, about 5 to 500 g/L, and preferably about 80 to 320 g/L.

In the present invention, a hydroxyl group-containing aromatic compound is used. Examples of hydroxyl group-containing aromatic compounds include compounds in which a benzene ring or a furan ring is substituted with one or more hydroxyl groups. Compounds having a benzene ring are preferable from the viewpoint of the work environment and solution stability. Specific examples of hydroxyl group-containing aromatic compounds include phenol, catechol, hydroquinone, resorcinol, pyrogallol, p-cresolsulfonic acid, ascorbic acid, erythorbic acid; alkali metal salts thereof; and the like. Examples of alkali metals include sodium, potassium, and the like. Preferred hydroxyl group-containing aromatic compounds are phenol, catechol, hydroquinone, resorcinol, pyrogallol, p-cresolsulfonic acid, sodium ascorbate, and sodium erythorbate. These hydroxyl group-containing aromatic compounds are considered to have the action of reducing divalent copper ions (Cu²⁺) to monovalent copper ions (Cu¹⁺), and are considered to aid copper ions and the complexing agent in forming a complex. These hydroxyl group-containing aromatic compounds can generally be used singly, or in a combination of two or more. The concentration of the hydroxyl group-containing aromatic compound is, for example, about 1 to 50 g/L, and preferably about 5 to 30 g/L.

The amounts of the complexing agent and the hydroxyl group-containing aromatic compound are such that relative to 1 mol/L of copper, the amount of the complexing agent is preferably 2 mol/L or more, and the amount of the hydroxyl group-containing aromatic compound is preferably 1 mol/L or more.

Examples of acids constituting the base of the copper-tin alloy plating bath include a wide variety of known organic acids and inorganic acids. Specific examples of organic acids include methanesulfonic acid, ethanesulfonic acid, 2-propanolsulfonic acid, 2-sulfoacetic acid, 2-sulfopropionic acid, 3-sulfopropionic acid, sulfosuccinic acid, sulfomethylsuccinic acid, sulfofumaric acid, sulfomaleic acid, 2-sulfobenzoic acid, 3-sulfobenzoic acid, 4-sulfobenzoic acid, 5-sulfosalicylic acid, 4-sulfophthalic acid, 5-sulfoisophthalic acid, 2-sulfoterephthalic acid, phenolsulfonic acid, and the like. Specific examples of inorganic acids include sulfuric acid, hydrochloric acid, sulfamic acid, and the like. Among these, sulfuric acid, methanesulfonic acid, sulfosuccinic acid, and the like are preferable. These acids can generally be used singly, or in a combination of two or more. The concentration of the acid is about 10 to 400 g/L, and preferably about 150 to 200 g/L.

The copper-tin alloy plating bath is generally in a weakly acidic to strongly acidic pH range. More specifically, the pH of the solution of the plating bath is adjusted to 4.5 or less. An overly high pH is not preferable because if the pH is overly high, the obtained plating film will have insufficient smoothness. Examples of usable pH adjusting agents include various acids, such as hydrochloric acid and sulfuric acid; various bases, such as ammonium hydroxide, sodium hydroxide, and potassium hydroxide; and the like. In addition, a pH buffer may be added to reduce variation in the pH of the plating bath. As the pH buffer, a known pH buffer can be used. Examples of pH buffers include sodium or potassium acetate, sodium, potassium, or ammonium borate, sodium or potassium formate, sodium or potassium tartrate, sodium, potassium, or ammonium dihydrogen phosphate, and the like. These pH adjusting agents and pH buffers can generally be used singly, or in a combination of two or more.

The plating bath may contain additives, such as high molecular compounds, surfactants, and levelers, if necessary.

Examples of high molecular compounds include polyethylene glycol and the like.

Examples of usable surfactants include known nonionic surfactants, cationic surfactants, anionic surfactants, and amphoteric surfactants. These surfactants can be used singly, or in a combination of two or more. It is preferable that at least one nonionic surfactant be contained.

Examples of nonionic surfactants include polyoxyalkylene alkyl ethers, polyoxyalkylene phenyl ethers, polyoxyalkylene naphthyl ethers, polyoxyalkylene alkyl esters, polyoxyalkylene sorbitan fatty acid esters, polyoxyalkylene sorbit fatty acid esters, polyethylene glycol fatty acid esters, polyoxyalkylene glycerin fatty acid esters, polyoxyalkylene alkylamines, and the like. Among these, polyoxyalkylene alkylamines are preferable, and polyoxyethylene alkylamines are more preferable. Examples of cationic surfactants include tetra-lower-alkylammonium halides, alkyltrimethylamnonium halides, alkylamine hydrochlorides, alkylamine oleates, alkylaminoethylglycines, and the like. Examples of anionic surfactants include alkyl-β-naphthalenesulfonic acids, fatty acid soap-based surfactants, alkyl sulfonic acid salts, alkyl sulfuric acid ester salts, polyoxyethylene alkylphenol ether sulfuric acid ester salts, and the like. Examples of amphoteric surfactants include 2-alkyl-N-carboxymethyl-N-hydroxyethyl imidazolinium betaines, dimethylalkylbetaines, sulfobetaines, N-alkyl-β-aminopropionic acids, and the like.

When a high molecular compound or a surfactant is added to the plating bath, the concentration of the high molecular compound or the surfactant can be in the range of about 0.01 to 100 g/L, and preferably about 0.1 to 40 g/L.

Levelers are additives that improve smoothness and brightness. Examples of usable levelers include ketone compounds and aldehyde compounds. As ketone compounds, a wide variety of known aromatic ketones and aliphatic ketones can be used. Examples of aromatic ketones include acetophenone, benzophenone, benzalacetone, and the like. Examples of aliphatic ketones include acetone, diethyl ketone, and the like. As aldehyde compounds, a wide variety of known aromatic aldehydes and aliphatic aldehydes can be used. Examples of aromatic aldehydes include cinnamaldehyde, α-methylcinnamaldehyde, α-amylcinnamaldehyde, α-hexylcinnamaldehyde, cuminaldehyde, benzaldehyde, anisaldehyde, and the like. Examples of aliphatic aldehydes include formaldehyde, acetaldehyde, propionaldehyde, and the like. Among these, aromatic ketones and aromatic aldehydes are preferable. These levelers can be used singly, or in a combination of two or more.

When a leveler is added to the plating bath, the concentration of the leveler can be in the range of about 0.01 to 30 g/L, and preferably about 0.01 to 10 g/L.

As additives, a surfactant and a leveler are preferably used in combination. A combined use of a surfactant and a leveler can expand the current-density region in which a bright plating film is obtained. This enables a plating film obtained from the plating bath of the present invention to have more excellent smoothness and higher brightness. As a combination of a surfactant and a leveler, a combination of a nonionic surfactant, and an aromatic ketone or an aromatic aldehyde is preferable. The nonionic surfactant is preferably a polyoxyethylene alkylamine. A plating film having excellent bright appearance can be obtained by further adding a nonionic surfactant, and an aromatic ketone or an aromatic aldehyde to the plating bath.

When a surfactant and a leveler are used in combination, the concentration of the surfactant is preferably about 0.1 to 40 g/L, the concentration of the leveler is preferably about 0.01 to 10 g/L, and the surfactant:leveler ratio is preferably about 1:1 to 100:1.

Additives other than the additives mentioned above, such as stress-reducing agents, conductive auxiliary agents, antifoaming agents, and brighteners, may be suitably selected and added to the plating bath, if necessary.

Examples of stress-reducing agents include naphtholsulfonic acid, saccharin, sodium 1,5-naphthalenedisulfonate, and the like. These can be used singly, or in a combination of two or more. Examples of conductive auxiliary agents include hydrochloric acid, sulfuric acid, acetic acid, nitric acid, sulfamic acid, pyrophosphoric acid, boric acid, and like acids; ammonium salts, sodium salts, potassium salts, and organic amine salts thereof; and the like. These can be used singly, or in a combination of two or more. As antifoaming agents and brighteners, commercially available antifoaming agents and brighteners for copper plating, tin plating, copper-tin alloy plating, and general plating can be suitably selected and used.

The bath preparation method for the plating bath of the present invention is not particularly limited. For example, the target plating solution can be obtained by dissolving a water-soluble copper compound and a water-soluble divalent tin compound in an aqueous solution in which an acid such as sulfuric acid is dissolved; adding a complexing agent and a reducing agent thereto; adding, if necessary, other additives thereto; and, finally, adjusting the pH to a predetermined pH.

There is no particular limitation on the plating method in which the plating bath of the present invention is used. The plating bath of the present invention can be used in known plating methods, and can also be applied to barrel plating, in which the variation in current density is large.

When the bath temperature during plating is low, throwing power increases, but the film formation speed tends to decrease. Conversely, when the bath temperature is high, the film formation speed increases, but throwing power onto low-current-density regions tends to decrease. Taking this point into consideration, an appropriate bath temperature can be determined. The bath temperature is preferably in the range of about 5 to 40° C.

The cathode current density can also be appropriately determined according to the plating solution used, type of object to be plated, etc. A cathode current density of about 0.1 to 3 A/dm² is preferable.

The anode may be any known anode that can be used for copper-tin alloy plating, such as a soluble anode (e.g., a tin anode, a phosphorus-containing copper anode, an oxygen-free copper anode, or a copper-tin alloy anode) or an insoluble anode (e.g., a stainless anode, a carbon anode, a lead anode, a lead-tin alloy anode, a lead-antimony alloy anode, a platinum anode, a titanium anode, a titanium-platinum anode, or an oxide coated anode, such as an iridium-oxide-coated titanium electrode). The cathode is an object to be plated that is described below. Thus, it can be said that the method for copper-tin alloy plating of the present invention is a method in which electrolysis is performed using an object to be plated as a cathode in the copper-tin alloy plating bath described above.

The copper-tin alloy plating film described above is formed on the surface of an article to be plated by the above plating method. The alloy composition of the obtained film is such that the Cu:Sn weight ratio is 95:5 to 5:95, and the alloy composition can be easily changed by varying the Cu concentration or the Sn concentration in the plating solution. The article to be plated is not particularly limited as long as the surface is conductive and smooth. Examples of such articles include home appliances, faucet fittings, sundry articles, decorations, clothing accessories, and the like.

The copper-tin alloy plating bath of the present invention can be suitably used for plating for clothing accessories or decorations; and plating for, for example, electronic or electric components. However, this does not limit applications to other purposes.

EXAMPLES

The present invention is described below in more detail with reference to Examples and Comparative Examples.

Plating treatment was performed using plating baths having the compositions shown in Tables 1 to 6 below under the following conditions to individually form plating films on objects to be plated.

Object to be plated: iron plate (5 cm×5 cm)

Plating Method:

-   -   Anode: pure tin plate (10 cm×5 cm, two plates)     -   Amount of solution: 1.5 L (a plastic container having a size of         14 cm×8 cm×18 cm was used)     -   Stirring: shaking with a cathode rocker

Plating Conditions:

-   -   Temperature: 18 to 20° C.     -   Current density: 1 A/dm²     -   Electrolytic time: 25 minutes

Tables 1 to 6 show the state of each plating solution, and the properties of each of the plating films formed as described above. The evaluation methods for the properties are as follows.

Solution state: The state of each solution was visually confirmed. Solution stability: After being allowed to stand for 24 hours, each plating solution was visually confirmed. Plating appearance and occurrence of cracking: Plating appearance and occurrence of cracking were observed with a digital microscope. Cu:Sn ratio: The Cu:Sn ratio was evaluated with a fluorescent X-ray film thickness measurement apparatus.

In addition, in the plating baths of Example 3 and Comparative Examples 11 and 12, plating treatment was performed at current densities of 0.01, 0.1, 0.5, 1, 2, and 3 A/dm², and the copper content of the formed plating films was determined. FIG. 1 shows the results.

TABLE 1 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 98% sulfuric acid (g/L) 185 185 185 — — 185 185 Methanesulfonic acid (g/L) — — — 150 — — — Sulfosuccinic acid (g/L) — — — — 150 — — Copper sulfate pentahydrate (g/L) 30 45 60 90 60 60 60 Stirrous sulfate (g/L) 10 10 10 10 10 10 10 3,6-dhia-1,8-octanediol (g/L) 60 90 120 120 120 — — Hydro(sodium sulfopropyl)dioxide (g/L) — — — — — 180 — Methanethiol (g/L) — — — — — — 90 1,2-ethanethiol (g/L) — — — — — — — 1,3-propanethiol (g/L) — — — — — — — Thioreses (g/L) — — — — — — — Sodium 3-encrycapto-1-propanesulfonate — — — — — — — (g/L) Thicyclodic acid (g/L) — — — — — — — Caresetol (g/L) 7 10 15 15 15 15 15 pH 0.1 or less 0.1 or less 0.1 or less 0.1 or less 0.1 or less 0.1 or less 0.1 or less Solution state No properties No properties No properties No properties No properties No Bad odor properties Solution stability Good Good Good Good Good Poor Poor Plating appeareance (1 A/dm²) Gray White White White White White Gray Dull Dull Dull Dull Dull Dull Dull Presence of absense of cracks No cracks No cracks No cracks No cracks No cracks No cracks No cracks Cu:Sn ratio (wt %) 43:57 51:49 54:30 63:37 66:34 59:41 66:35 Comparative Comparative Comparative Comparative Comparative Example 8 Example 9 Example 1 Example 2 Example 3 Example 4 Example 5 98% sulfuric acid (g/L) 185 185 185 185 185 185 185 Methanesulfonic acid (g/L) — — — — — — — Sulfosuccinic acid (g/L) — — — — — — — Copper sulfate pentahydrate (g/L) 60 60 60 60 60 60 60 Stirrous sulfate (g/L) 10 10 10 10 10 10 10 3,6-dhia-1,8-octanediol (g/L) — — — 120 — — — Hydro(sodium sulfopropyl)dioxide (g/L) — — — — — — — Methanethiol (g/L) — — — — — — — 1,2-ethanethiol (g/L) 60 — — — — — — 1,3-propanethiol (g/L) — 70 — — — — — Thioreses (g/L) — — — — 60 — — Sodium 3-encrycapto-1-propanesulfonate — — — — — 100 — (g/L) Thicyclodic acid (g/L) — — — — — — 60 Caresetol (g/L) 15 15 15 — 15 15 15 pH 0.1 or less 2 or less 0.1 or less 0.1 or less 0.1 or less 0.1 or less 0.1 or less Solution state Bad odor Bad odor Properties No properties Properties Bad odor Properties Solution stability Poor Poor Poor Poor Poor Poor Poor Plating appeareance (1 A/dm²) Gray Gray Plating were not performed. Dull Dull Presence of absense of cracks No cracks No cracks Cu:Sn ratio (wt %) 51:39 54:34

TABLE 2 Example 10 Example 11 Example 12 Example 13 Example 14 Example 15 98% sulfuric acid (g/L) 185 185 185 185 185 185 Copper sulfate pentahydrate (g/L) 60 60 60 60 60 60 Stirrous sulfate (g/L) 10 10 10 10 10 10 3,6-dhia-1,8-octanediol (g/L) 120 120 120 120 120 120 Hydroquinone (g/L) 15 — — — — — Resorcinol (g/L) — 15 — — — — Phenol (g/L) — — 15 — — β-cresolsulfuric acid (g/L) — — — 30 — — Pyrogalci (g/L) — — — — 20 Sodium ascorbate (g/L) — — — — — 30 Sodium erythorbate (g/L) — — — — — — Sodium hypoposphate (g/L) — — — — — — Sodium sulfate (g/L) — — — — — — Sodium hydrogen sulfate (g/L) — — — — — — Ammonium sulfate (g/L) — — — — — — Sodium hydro sulfate (g/L) — — — — — — pH 0.1 or less 0.1 or less 0.1 or less 0.1 or less 0.1 or less 0.1 or less Solution state No properties No properties No properties No properties No properties No properties Solution stability Good Good Good Good Good Good Plating appearence (1 A/dm²) Grey Grey Grey White Black White Dull Dull Dull Dull Dull Dull Presence of absense of cracks No cracks No cracks No cracks No cracks No cracks No cracks Cu:Sn ratio (wt %) 62:38 61:39 66:35 59:41 61:39 62:38 Comparative Comparative Comparative Comparative Comparative Example 16 Example 6 Example 7 Example 8 Example 9 Example 10 98% sulfuric acid (g/L) 185 185 185 185 185 185 Copper sulfate pentahydrate (g/L) 60 60 60 60 60 60 Stirrous sulfate (g/L) 10 10 10 10 10 10 3,6-dhia-1,8-octanediol (g/L) 120 120 120 120 120 120 Hydroquinone (g/L) — — — — — — Resorcinol (g/L) — — — — — — Phenol (g/L) — — — — — — β-cresolsulfuric acid (g/L) — — — — — — Pyrogalci (g/L) Sodium ascorbate (g/L) — — — — — — Sodium erythorbate (g/L) 30 — — — — — Sodium hypoposphate (g/L) — 15 — — — — Sodium sulfate (g/L) — — 20 — — — Sodium hydrogen sulfate (g/L) — — — 20 — — Ammonium sulfate (g/L) — — — — 20 — Sodium hydro sulfate (g/L) — — — — — 25 pH 0.1 or less 0.1 or less 2 or less 0.1 or less 0.1 or less 0.1 or less Solution state No properties No properties Bad odor Bad odor Bad odor Properties Solution stability Good Poor Good Good Good Poor Plating appearence (1 A/dm²) Grey Grey Plating were not performed. Dull Dull Presence of absense of cracks No cracks Cracks Cu:Sn ratio (wt %) 63:37 62:38

TABLE 3 Example 17 Example 18 Example 19 Example 20 Example 21 Example 22 98% sulfuric acid (g/L) 185 185 185 185 185 185 Copper sulfate pentahydrate (g/L) 60 60 60 60 60 60 Stannous sulfate (g/L) 10 10 10 10 10 10 3,6-dhia-1,8-octanediol (g/L) 120 120 120 120 120 120 Polyoxyethylene alkylamine (g/L) 10 — — — — — Polyethylene glycol (g/L) — 5 — — — — Polyoxyalkylene phenyl ether (g/L) — — 20 — — — Polyoxyalkylene naphthyl ether (g/L) — — — 10 — — Tetra-lower-alkylammoniumhalide (g/L) — — — — 0.5 — Alkylamine hydrochloride (g/L) — — — — — 1 Alkylamine thyglycho (g/L) — — — — — — Alkyl-β-naphinalenesulfuric acid (g/L) — — — — — — Fatty acid soap based sulfudants (g/L) — — — — — — Phenolether sulfuric acid estor acid (g/L) — — — — — — Sulfobetaine (g/L) — — — — — — Dimethylalkybetaine (g/L) — — — — — — pH 0.1 or less 0.1 or less 0.1 or less 0.1 or less 0.1 or less 0.1 or less Solution state No properties No properties No properties No properties No No properties properties Solution stability Good Good Good Good Good Good Plating appearence (1 A/dm²) White White White White White White Semi-bright Semi-bright Semi-bright Semi-bright Semi-bright Semi-bright Presence of absense of cracks No cracks No cracks No cracks No cracks No cracks No cracks Cu:Sn ratio (wt %) 61:39 58:42 63:37 59:41 60:40 57:43 Example 23 Example 24 Example 25 Example 26 Example 27 Example 28 98% sulfuric acid (g/L) 185 185 185 185 185 185 Copper sulfate pentahydrate (g/L) 60 60 60 60 60 60 Stannous sulfate (g/L) 10 10 10 10 10 10 3,6-dhia-1,8-octanediol (g/L) 120 120 120 120 120 120 Polyoxyethylene alkylamine (g/L) — — — — — — Polyethylene glycol (g/L) — — — — — — Polyoxyalkylene phenyl ether (g/L) — — — — — — Polyoxyalkylene naphthyl ether (g/L) — — — — — — Tetra-lower-alkylammoniumhalide (g/L) — — — — — — Alkylamine hydrochloride (g/L) — — — — — — Alkylamine thyglycho (g/L) 1 — — — — — Alkyl-β-naphinalenesulfuric acid (g/L) — 30 — — — — Fatty acid soap based sulfudants (g/L) — — 10 — — — Phenolether sulfuric acid estor acid (g/L) — — — 5 — — Sulfobetaine (g/L) — — — — 5 — Dimethylalkybetaine (g/L) — — — — — 10 pH 0.1 or less 0.1 or less 1 or less 0.1 or less 0.1 or less 0.1 or less Solution state No properties No properties No properties No properties No No properties properties Solution stability Good Good Good Good Good Good Plating appearence (1 A/dm²) White White White White White White Semi-bright Semi-bright Semi-bright Semi-bright Semi-bright Semi-bright Presence of absense of cracks No cracks No cracks No cracks No cracks No cracks No cracks Cu:Sn ratio (wt %) 63:37 53:37 58:42 59:41 55:44 63:37

TABLE 4 Example 29 Example 30 Example 31 Example 32 Example 33 Example 34 98% sulfuric acid (g/L) 185 185 185 185 185 185 Copper sulfate pentahydrate (g/L) 60 60 60 60 60 60 Stirrous sulfate (g/L) 10 10 10 10 10 10 3,6-dhia-1,8-octanediol (g/L) 120 120 120 120 120 120 Benzalacetone (g/L) 1 — — — — — Cinnarrealdehyde (g/L) — 1 — — — — α-Methylcinnarrealdehyde (g/L) — — 3 — — — α-hexylcinnarrealdehyde (g/L) — — — 5 — — α-anhycinnarrealdehyde (g/L) — — — — 5 — Cuminaldehyde (g/L) — — — — — 1 Benzaldehyde (g/L) — — — — — — Anicaldehyde (g/L) — — — — — — Propionalaldehyde (g/L) — — — — — — Acetaldehyde (g/L) — — — — — — Formoldehyde (g/L) — — — — — — pH 0.1 or less 0.1 or less 0.1 or less 0.1 or less 0.1 or less 0.1 or less Solution state No properties No properties No properties No properties No properties No properties Solution stability Good Good Good Poor Poor Good Plating appearence (1 A/dm²) White White White White White White Semi-bright Semi-bright Semi-bright Semi-bright Semi-bright Semi-bright Presence of absense of cracks No cracks No cracks No cracks No cracks No cracks No cracks Cu:Sn ratio (wt %) 57:43 58:42 60:40 58:42 57:43 61:39 Example 35 Example 36 Example 37 Example 38 Example 39 98% sulfuric acid (g/L) 185 185 185 185 185 Copper sulfate pentahydrate (g/L) 60 60 60 60 60 Stirrous sulfate (g/L) 10 10 10 10 10 3,6-dhia-1,8-octanediol (g/L) 120 120 120 120 120 Benzalacetone (g/L) — — — — — Cinnarrealdehyde (g/L) — — — — — α-Methylcinnarrealdehyde (g/L) — — — — — α-hexylcinnarrealdehyde (g/L) — — — — — α-anhycinnarrealdehyde (g/L) — — — — — Cuminaldehyde (g/L) — — — — — Benzaldehyde (g/L) 1 — — — — Anicaldehyde (g/L) — 3 — — — Propionalaldehyde (g/L) — — 3 — — Acetaldehyde (g/L) — — — 0.5 — Formoldehyde (g/L) — — — — 0.5 pH 0.1 or less 0.1 or less 1 or less 0.1 or less 0.1 or less Solution state No properties No properties No properties No properties No properties Solution stability Good Good Good Good Good Plating appearence (1 A/dm²) White White White White White Semi-bright Semi-bright Semi-bright Semi-bright Semi-bright Presence of absense of cracks No cracks No cracks No cracks No cracks No cracks Cu:Sn ratio (wt %) 58:42 61:39 59:41 63:37 64:36

TABLE 5 Example 40 Example 41 Example 42 Example 43 Example 44 Example 45 98% sulfuric acid (g/L) 185 185 185 185 185 185 Copper sulfate pentahydrate (g/L) 60 60 60 60 60 60 Stirrous sulfate (g/L) 10 10 10 10 10 10 3,6-dhia-1,8-octanediol (g/L) 120 120 120 120 120 120 Polyoxyethylene alkylamine (g/L) 10 10 10 10 10 10 Bezalacetone (g/L) 1 — — — — — Cinnarrealdehyde (g/L) — 1 — — — — α-Methylcinnarrealdehyde (g/L) — — 3 — — — α-hexylcinnarrealdehyde (g/L) — — — 5 — — α-anhycinnarrealdehyde (g/L) — — — — 5 — Cuminaldehyde (g/L) — — — — — 1 Benzaldehyde (g/L) — — — — — — Anicaldehyde (g/L) — — — — — — Propionalaldehyde (g/L) — — — — — — Acetaldehyde (g/L) — — — — — — Formoldehyde (g/L) — — — — — — pH 0.1 or less 0.1 or less 0.1 or less 0.1 or less 0.1 or less 0.1 or less Solution state No properties No properties No properties No properties No properties No properties Solution stability Good Good Good Poor Poor Good Plating appearence (1 A/dm²) White White White White White White Bright Bright Bright Bright Bright Bright Presence of absense of cracks No cracks No cracks No cracks No cracks No cracks No cracks Cu:Sn ratio (wt %) 59:41 58:42 61:39 57:43 62:38 58:42 Example 46 Example 47 Example 48 Example 49 Example 50 98% sulfuric acid (g/L) 185 185 185 185 185 Copper sulfate pentahydrate (g/L) 60 60 60 60 60 Stirrous sulfate (g/L) 10 10 10 10 10 3,6-dhia-1,8-octanediol (g/L) 120 120 120 120 120 Polyoxyethylene alkylamine (g/L) 10 10 10 10 10 Bezalacetone (g/L) — — — — — Cinnarrealdehyde (g/L) — — — — — α-Methylcinnarrealdehyde (g/L) — — — — — α-hexylcinnarrealdehyde (g/L) — — — — — α-anhycinnarrealdehyde (g/L) — — — — — Cuminaldehyde (g/L) — — — — — Benzaldehyde (g/L) 1 — — — — Anicaldehyde (g/L) — 3 — — — Propionalaldehyde (g/L) — — 3 — — Acetaldehyde (g/L) — — — 0.5 — Formoldehyde (g/L) — — — — 0.5 pH 0.1 or less 0.1 or less 1 or less 0.1 or less 0.1 or less Solution state No properties No properties No properties No properties No properties Solution stability Good Good Good Good Good Plating appearence (1 A/dm²) White White White White White Bright Bright Bright Bright Bright Presence of absense of cracks No cracks No cracks No cracks No cracks No cracks Cu:Sn ratio (wt %) 58:42 62:38 59:41 58:41 62:38

TABLE 6 Comparative Comparative Example 11 Example 12 Potassium pyrophosphate (g/L) 200 — Copper pyrophosphate (g/L) 20 — Stannous sulfate (g/L) 10 — Organic sulfonic acid (g/L) — 100 Stannous sulfate (g/L) — 36 Copper sulfate pentahydrate (g/L) — 12 Brightener Appropriate Appropriate amount amount pH 7 to 8 0.5 Solution state No precipitates No precipitates Solution stability Good Poor Plating appearance (1 A/dm²) White White Presence or absence of cracks Cracks No cracks Cu:Sn ratio (wt %) 58:42 53:47

The results of Tables 1 to 5 reveal that no precipitates were formed in the plating baths of Examples 1 to 50; that the solution state was stable, especially in the plating baths of Examples 1 to 5, 10 to 31, 34 to 42, and 45 to 50; and that crack-free plating films were obtained by plating. As is clear from the results of Examples 1 to 5 in Table 1, a copper-tin alloy plating film having any ratio can be obtained by adjusting the metal concentration in the plating solution. The results of Tables 3 to 5 show that adding a surfactant or a leveler to the plating solution improves brightness of a plating appearance, and that a plating appearance having excellent brightness can be obtained by adding both a surfactant and a leveler to the plating solution. In addition, FIG. 1 shows that in the plating bath of the present invention, the current density has a small influence on the alloy ratio as compared with the case of a hitherto known acidic bath (Comparative Example 12). 

1. A copper-tin alloy plating bath comprising an aqueous solution containing a water-soluble copper compound, a water-soluble divalent tin compound, a sulfur-containing compound represented by formula (1): R—(CH₂)_(l)—S—(CH₂)_(m)—S—(CH₂)_(n)—R  (1), wherein R is H, OH, or SO₃Na, and l, m, and n are each independently an integer of 0 to 3, and a hydroxyl group-containing aromatic compound.
 2. The copper-tin alloy plating bath according to claim 1, wherein the water-soluble copper compound is present in an amount such that the amount of copper ions is 1 to 60 g/L, the water-soluble divalent tin compound is present in an amount such that the amount of divalent tin ions is 5 to 40 g/L, the sulfur-containing compound is present in an amount of 5 to 500 g/L, and the hydroxyl group-containing aromatic compound is present in an amount of 1 to 50 g/L.
 3. The copper-tin alloy plating bath according to claim 1, wherein the sulfur-containing compound is at least one member selected from the group consisting of methanedithiol, 1,2-ethanedithiol, 1,3-propanedithiol, 3,6-dithia-1,8-octanediol, and bis-(sodium sulfopropyl)-disulfide.
 4. The copper-tin alloy plating bath according to claim 1, wherein the hydroxyl group-containing aromatic compound is at least one member selected from the group consisting of phenol, catechol, hydroquinone, resorcinol, pyrogallol, p-cresolsulfonic acid, sodium ascorbate, and sodium erythorbate.
 5. The copper-tin alloy plating bath according to claim 1, wherein the aqueous solution further contains a nonionic surfactant, and an aromatic ketone or an aromatic aldehyde.
 6. A method for copper-tin alloy plating, the method comprising performing electrolysis using an object to be plated as a cathode in the copper-tin alloy plating bath according to claim
 1. 7. An article comprising a copper-tin alloy plating film formed by the method according to claim
 6. 8. The copper-tin alloy plating bath according to claim 2, wherein the sulfur-containing compound is at least one member selected from the group consisting of methanedithiol, 1,2-ethanedithiol, 1,3-propanedithiol, 3,6-dithia-1,8-octanediol, and bis-(sodium sulfopropyl)-disulfide.
 9. The copper-tin alloy plating bath according to claim 2, wherein the hydroxyl group-containing aromatic compound is at least one member selected from the group consisting of phenol, catechol, hydroquinone, resorcinol, pyrogallol, p-cresolsulfonic acid, sodium ascorbate, and sodium erythorbate.
 10. The copper-tin alloy plating bath according to claim 3, wherein the hydroxyl group-containing aromatic compound is at least one member selected from the group consisting of phenol, catechol, hydroquinone, resorcinol, pyrogallol p-cresolsulfonic acid, sodium ascorbate, and sodium erythorbate.
 11. The copper-tin alloy plating bath according to claim 2, wherein the aqueous solution further contains a nonionic surfactant, and an aromatic ketone or an aromatic aldehyde.
 12. The copper-tin alloy plating bath according to claim 3, wherein the aqueous solution further contains a nonionic surfactant, and an aromatic ketone or an aromatic aldehyde.
 13. The copper-tin alloy plating bath according to claim 4, wherein the aqueous solution further contains a nonionic surfactant, and an aromatic ketone or an aromatic aldehyde.
 14. A method for copper-tin alloy plating, the method comprising performing electrolysis using an object to be plated as a cathode in the copper-tin alloy plating bath according to claim
 2. 15. A method for copper-tin alloy plating, the method comprising performing electrolysis using an object to be plated as a cathode in the copper-tin alloy plating bath according to claim
 3. 16. A method for copper-tin alloy plating, the method comprising performing electrolysis using an object to be plated as a cathode in the copper-tin alloy plating bath according to claim
 4. 17. A method for copper-tin alloy plating, the method comprising performing electrolysis using an object to be plated as a cathode in the copper-tin alloy plating bath according to claim
 5. 